This invention relates to a process for fabricating thin film transistors.
Thin film transistors (TFT's) are known to be useful for, inter alia, controlling various types of display; for example TFT's are commonly used to control liquid crystal displays used in portable computers and similar electronic devices. TFT's can also be used to control electrophoretic displays; see, for example, Applications Ser. Nos. 60/132,642, filed May 5, 1999 and Ser. No. 09/565,413, filed May 5, 2000 (now U.S. Pat. No. 7,030,412); Applications Ser. Nos. Ser. No. 60/144,943, filed Jul. 21, 1999, 60/147,989, filed Aug. 10, 1999, and Ser. No. 09/621,000, filed Jul. 22, 2000 (now U.S. Pat. No. 6,842,657); Applications Ser. Nos. 60/151,547, filed Aug. 31, 1999, and 09/652,486, filed Aug. 31, 2000 (now U.S. Pat. No. 6,312,971); Applications Ser. Nos. 60/151,715 and 60/151,716, both filed Aug. 31, 1999, and 09/651,710, filed Aug. 31 2000 (now U.S. Pat. No. 6,498,114); and Applications Ser. Nos. 60/151,549, filed Aug. 31, 1999 and Ser. No. 09/650,620, filed Aug. 30, 2000 (now U.S. Pat. No. 6,545,291). The disclosures of all these copending applications are herein incorporated by reference. See also the corresponding International Applications Publication Nos. WO 00/67327; WO 01/08241;WO 01/17029; WO 01/17040; and WO 01/17041.
Although most TFT's have hitherto been fabricated on rigid substrates, there is increasing interest in fabricating TFT's on flexible substrates, especially flexible polymeric films. TFT's fabricated on such flexible substrates could form the basis for large displays which would be light-weight yet rugged, thus permitting their use in mobile devices. TFT's based upon amorphous silicon semiconductors are attractive for use on such flexible substrates since they allow fabrication with a minimum number of process steps and with a low thermal budget. Amorphous silicon transistors have been fabricated on ultra-thin stainless steel substrates (see, for example, Ma et al., Applied Physics Letters, 74(18), 2661 (1999)) and on polyimide films (see Gleskova et al., IEEE Electron Device Letters, 20(9), 473 (1999)).
However, the polyimide used in the process described in the latter paper, sold commercially under the name “Kapton” (Registered Trade Mark) has a glass transition temperature of only about 300° C., which restricts the temperatures which can be employed during the fabrication process, and results in a less satisfactory amorphous silicon semiconductor layer. This polyimide also has a high moisture absorption (about 4 percent by weight) and such high moisture absorption can result in swelling of the substrate and consequent cracking of thin layers deposited on the substrate, or delamination of thin layers from the substrate. Although stainless steel substrates can withstand process temperatures much higher than 300° C., such substrates require both passivation and planarization steps before transistors can be fabricated thereon. Passivation is required to ensure proper electrical isolation between adjoining metal conductors to be formed on the substrate, and to ensure that potential contaminants within the stainless steel do not diffuse into the transistors. Stainless steel substrates do, however, have the advantages of high dimensional stability and ease of handling in a manufacturing environment.
It has now been discovered that certain types of polyimides possess properties which render them very suitable for use as substrates in the fabrication of TFT's. These polyimide substrates may be used with or without a metal backing layer.